The present invention relates to a process for producing polyethylene, and in particular a process producing medium density polyethylene using a chromium-based catalyst.
Polyethylene is known for use in the manufacture of a wide variety of articles. The polyethylene polymerisation process can be varied in a number of respects to produce a wide variety of resultant polyethylene resins having different physical properties which render the various resins suitable for use in different applications. Medium density polyethylene resins are known for use in making films. Such medium density films are known to have good resin processability due to the presence of long chain branching in the polyethylene polymer molecules. It is known to produce such resins using chromium-based catalysts, which have been known for some time. Unfortunately, some medium density resins produced by such catalysts suffer from the problem that when the film is to be used for the packaging of foods for human consumption, it is required that the level of extractable or volatile compounds in the polyethylene resin is low so that the food is not inadvertently contaminated. For example, in the United States the Food and Drug Administration has set maximum limits for the amount of extractable or volatile compounds in polyethylene resins for food applications.
It is also known to produce polyethylene using metallocene catalysts, typically unbridged metallocene catalysts. Such catalysts are known to produce polyethylene resin films having low values of extractable components due to the narrow molecular weight distribution in the polyethylene resin, which yields a very low proportion of components in the resin at low molecular weight. Unfortunately, such films suffer from the technical problem that the combination of narrow molecular weight distribution and the absence of long chain branching are responsible for the resins having poor processability.
U.S. Pat. No. 5,284,926 discloses an ethylene polymerisation catalyst system and process. It is disclosed that the catalyst may be a chromium-based catalyst which may be subjected to fluorination and subsequent reduction by carbon monoxide. The polymerization process may be carried out in the presence of a trialkyl aluminium co-catalyst. However, there is no disclosure of the production of medium density polyethylene resins, or in particular the production of such resins suitable for food packaging having a low level of extractable or volatile compounds in the resin.
U.S. Pat. No. 5,352,658 discloses a similar catalyst system and polymerization process. This specification addresses the problem of providing excellent low temperature impact strength and preventing swelling in the blow moulding process on emergence from the extruder die. The specification does not address the problem of the production of medium density polyethylene films having suitability for food packaging by having low levels of extractable or volatile compounds.
EP-A-0548805 discloses polymerization catalysts and processes for the production of high density polyethylene homopolymers. There is no disclosure of the production of medium density polyethylene resins suitable for use in making film packaging for food.
U.S. Pat. No. 5,071,927 discloses high-temperature slurry polymerization of ethylene using a chromium-based catalyst and a trialkyl boron or polyalkyl silane co-catalyst. Although the production of a medium density polyethylene resin is disclosed, there is no disclosure of such a resin having low levels of extractable or volatile compounds to render the resin suitable for food packaging.